1. Field of the Invention
The present invention relates to an electromagnetic ultrasonic transducer for coupling media free generation and/or reception of ultrasonic waves in the form of linearly polarized transverse waves into, and from, a workpiece having at least one unit that converts the ultrasonic waves inside the workpiece. The unit has a coil arrangement for generating, and/or detecting, a high-frequency (HF) magnetic field and a premagnetizing unit for generating a quasi-static magnetic field which superimposes the HF magnetic field in the work piece, with the coil arrangement being torus shaped on at least one partially toroidal or U-shaped magnetic core which has two front ends which can each be turned to face the workpiece.
These ultrasonic probes permit generating and receiving linearly polarized transverse waves, which preferably are irradiated under the probe perpendicularly into the workpiece, and are received from this direction and oscillate preferably perpendicular to their propagation direction. Technical fields of application of such type ultrasonic probes are, for example, nondestructive examination of electrically conductive workpieces for material flaws, such as for example cracks, in particular crack-like flaws oriented parallel to the polarization direction of the ultrasonic waves and perpendicular to the propagation direction, as well as other processes based on ultrasonic velocity and polarization, such as for example measuring voltage or, in particular, measuring thickness.
2. Description of the Prior Art
The coupling media free electromagnetic probes known in the art convert electromagnetic field energies into elastic energy of an ultrasonic wave and inversely. The conversion mechanism is based on the interaction between the electromagnetic field and an electrically conducting material that also has a static magnetic field or a quasi-static magnetic field applied from the outside which passes through the conducting material. The term “quasi-static” magnetic field comprises, in addition to the actual static magnetic field, which for example can be generated by means of permanent magnets, also low-frequency magnetic fields, whose alternating frequency is much lower than the high frequency with which the coil arrangement is operated to generate high-frequency fields.
In order to excite ultrasonic waves inside an electrically conducting workpiece, at least one part of the high-frequency magnetic field, whose frequency range lies within the ultrasonic frequency range, generated by the high-frequency coil arrangement, is coupled into the workpiece, thus inducing eddy currents at skin depth, which if superimposed on the “quasi-static” magnetic field, generate ultrasonic waves due to the Lorentz forces or magnetostrictions occurring inside the workpiece.
Detection of ultrasonic waves occurring inside the workpiece occurs inversely by detection of the electrical voltage induced inside the coil arrangement resulting from high-frequency fields which for their part are generated by the motions of electric charges, due to the ultrasonic waves, in the workpiece inside the “quasi-static” magnetic field.
All prior art electromagnetic ultrasonic transducers are based on optimizing measuring sensitivity and, related thereto, the signal amplitudes of both the transmission signal and in the reception signal that can be generated with the coil arrangements. The goal, on the one hand, is to design the coupling mechanism with which the generated and to-be-detected high-frequency fields are coupled into and out of the ultrasonic transducer and the workpiece as loss-free as possible and, on the other hand, to select the field strength of the quasi-static magnetic field as large as possible, which is decisive for generating and detecting ultrasonic waves.
German Patent DE 42 23 470 C2 describes a generic electromagnetic probe for vertical acoustic irradiation of linearly polarized transverse waves, in which the high-frequency magnetic fields are coupled into and out of in a most efficient manner between the probe and the workpiece without, as is the case with many other probes, placing the transmission and reception coils (usually designed as high-frequency air coils) directly on the surface of the workpiece. But rather the electromagnetic probe of FIG. 2 described in this printed publication is provided with a half-open toroidal tape core 1, made commercially of amorphous tape material, around which a transmission coil 41 and a reception coil 42, respectively, are wound. The front ends 2 of the half-open toroidal core 1 act as coupling areas for the high-frequency magnetic fields and can be placed on the surface of the workpiece 7. The high-frequency magnetic fields generated by the high-frequency transmission coil arrangement 41 reach, via the front ends 2 of the toroidal core 1, into the workpiece 7 and are able to induce the surface eddy currents 8 at skin depth inside the workpiece 7.
The quasi-static magnetic field oriented perpendicular to the surface of the workpiece 7 required for sound conversion is generated by means of two permanent magnets 6 of the same name and conveyed to the material surface of the workpiece 7. The premagnetizing unit required for producing the “quasi-static” magnetic field that is oriented perpendicular to the surface of the workpiece is located inside the open part of the toroidal core 1. With this arrangement, ultrasonic waves with a propagation direction A perpendicular to the surface of the workpiece and an oscillation plane S perpendicular thereto develop inside the workpiece.
German Patent DE 41 30 935 A1 describes a probe device comparable to this arrangement. However, in this probe device the transmission and reception coil arrangement lies directly on the surface of the workpiece to-be-examined, which has the danger of coil wear.
German Patent DE 195 43 482 A1 describes a device for testing ferromagnetic materials, preferably in the form of pipe lines. However, this device has a component setup that differs from the state of the art described in detail in the preceding and on which the following is based.